64 



THE MECHANISM OF LIFE 



the liver. Something is done to the blood there, and then the' 

 capillaries reunite into veins and then into another great vessel] 

 the hepatic vein. 



At the same time the blood which has been circulating in the: 

 capillaries of the muscles of the hind-limbs and trunk, in thd 

 kidneys, reproductive organs, spleen in short, ia all the body ; 

 behind the upper limbs is gathered up into a vein called the^ 

 posterior vena cava, and this is joined by the hepatic vein. All 

 the blood that has been circulating in the fore-limbs, the chest, 

 head, and neck is collected by two veins called the anterior vena- 

 cavce. Finally, all three caval veins unite and pour their blood 

 into the heart. , 



To muscles of- 



From muse let / 

 of trunk 



FIG. 14. DIAGRAM OF THE BLOODVESSELS OF A FISH. 

 i, ii, iii and iv are the gills. 



Thus there is a vascular system, which consists of a series of 

 veins returning all blood that has been circulating anywhere in 

 the body to the heart. Clearly there are at least two kinds of 

 blood in the veins: (1) That which has been circulating in the 

 muscles, and which has, therefore, been depleted of some of its 

 nutritive properties; and (2) that which has been circulating in 

 the intestine, and which has become enriched with substances of 

 nutritive value. Now we have to consider the other half of the 

 blood vascular system the arterial vessels which distribute this 

 nutritive substance received from the alimentary canal (via the' 

 liver) to the body at large. 



First of all we look at the arterial vessels of a fish (for here 

 the conditions are very simple). Much the same kind of venous 

 system is present in the fish as in man (or at least we need not 

 worry about the minor differences), but the arteries are not 

 complicated by the presence of lungs. 





